Motor-unit recruitment in the trapezius muscle during arm movements and in VDU precision work

It has been hypothesized that shoulder myalgia, which is common in many occupations involving light-to-moderate manual work, is due to an overuse of low threshold muscle fibers, causing damage at the muscle cell level. The present study was undertaken to investigate in the trapezius muscle, (a) to what extent it is possible to track low threshold motor-units in non-isometric conditions, and (b) to study the effect of arm movements on recruitment and firing patterns. Intramuscular four lead fine wire electrodes were inserted in the trapezius muscle of four volunteers, who performed arm movements (shoulder abduction–adduction 0–90–0° in the scapula plane), as well as standardized work using computer mouse. Three channels of electromyography (EMG) were used for motor-unit action potential (MUAP) identification and signal decomposition. Results showed (a) that it was possible to perform signal tracking during slow movements, and (b) that there were motor-units that were active over a wide range of shoulder abduction movement, as well as in work with computer mouse. The findings indicate that stereotypic recruitment patterns exist in the trapezius muscle not only in static work, but also in work situations involving arm and shoulder movements.

Relevance to industry

Occupationally related chronic shoulder pain is an increasing problem in industry, in manual work as well as in computer work. This paper presents results of studies on muscular activity patterns during movements which support the notion that chronic pain may be evoked by overuse syndromes at the muscular fiber level. Practical precautions to reduce such risks are discussed.     

Electromyographic assessment of apple bucket intervention designed to reduce back strain

The authors previously developed an apple bucket that was modified by use of a hip belt to reduce muscle fatigue. The intervention of belt use was accepted by workers and shown not to interfere with productivity. However, use of this intervention did not appear to reduce muscle fatigue when measured by tests of voluntary muscle strength. The purpose of the present study was to evaluate the intervention's effect on muscle fatigue employing surface electromyographic (EMG) amplitude. Amplitude measurements on 15 muscles were taken from 10 laboratory volunteers who were carrying a full bucket of apples, once while wearing the intervention belt and once without the intervention. These measurements were taken for seven different postures (four angles of trunk flexion (0 degrees , 20 degrees , 45 degrees , 90 degrees ) and three raised-arm positions (both up, dominant up, non-dominant up)) common to apple harvest work. Participants were measured in these conditions both with the bucket carried in front and with the bucket carried to the side. Significant reductions in amplitude favouring the intervention were seen for 11 of the 15 muscles in models considering the four body flexion angles. Ten of these were of the middle and lower back. These control/intervention differences were seen with both bucket-carrying positions (front vs. side) and tended to increase with increasing flexion angle. In contrast, no significant intervention effects were observed in models considering treatment by arm-raised position. One significant main effect (upper trapezius, side bucket) showed an amplitude reduction in the treatment condition. Another main effect showing increased amplitude in the intervention condition use was observed in the dominant levator scapulae (side bucket). Thus, the use of the intervention belt reduces EMG amplitude among a number of mid- and lower-back muscles. This is suggestive of a protective effect against back strain.     

An evaluation of backpack harness systems in non-neutral torso postures

Much of the research on backpack design has been focused on spinal loading/biomechanics while the wearer is in a neutral/upright trunk posture, such as those employed by outdoor enthusiasts and schoolchildren. This research has led to some important harness design improvements that reduce trunk muscle exertions, fatigue and improve overall comfort. There are number of occupations, however, wherein workers wear back-mounted packs/devices (e.g. air tanks) while working in non-neutral trunk postures. The objective of the current study was to evaluate the effects of these non-neutral postures on biomechanical loading and then reconsider the backpack system design recommendations. Fifteen participants were asked to support a 18.2 kg load on their back while assuming static forward flexed postures of the torso (15 degrees , 30 degrees , 45 degrees , and 60 degrees of sagittal bend). The mass on the back was attached to the participant through two different harness mechanisms: a basic harness design (as seen on college student backpacks) and a more advanced design containing lateral stiffness rods and a weight-bearing hip belt (as seen on backpacks for hikers). While performing these static, posture maintenance tasks, the activation levels of the bilateral trapezius, erector spinae, and rectus abdominis were collected. Participants also provided subjective ratings of comfort. The results showed that there was a significant interaction between harness type and forward flexion angle for the trapezius and the erector spinae muscles. The normalized EMG for the trapezius muscles showed a 14% and 11% reduction in muscle activity at 15 degrees and 30 degrees , respectively, with the advanced design but these positive effects of the advanced design were not found at the greater flexion angles. Likewise the erector spinae muscles showed a 24% and 14% reduction in muscle activity at 15 degrees and 30 degrees , respectively, with the advanced design harness but these effects of the advanced design were not found at the greater forward flexion angles. The level of forward flexion angle affected the rectus abdominis muscle activity, but neither the harness type main effect nor the interaction of harness type and forward flexion angle was significant. The subjective survey results agreed with the EMG results and showed the advanced design harness was generally more comfortable with respect to the shoulder and low back areas. Collectively, the subjective and objective results show a significant improvement with the advanced harness system but also note an interesting interaction with degree of sagittal flexion, indicating a diminished effectiveness of the design improvements at forward flexed postures. Design criteria for harness systems in these forward flexed postures are discussed.     

Work load and fatigue in repetitive arm elevations

Six females performed continuous series of concentric and eccentric flexions in the shoulder between 0 and 90 degrees with 0 to 3.1 kg weights held in a powergrip. Heart rate (H R), perceived exertion (RPE) and myoelectric activity (EMG) from the descending part of the trapezius muscle, the anterior part of the deltoid muscle, and the biceps brachii were measured during the tasks. The increase of RPE was faster than the increase of HR with work load indicating an increased importance of local factors (i.e. strain on muscles and tendons) with load in the perceived exertion. The local muscular load determined by EMG on the trapezius muscle was closely correlated with the external torque produced in the glenohumeral joint. The time constants of EMG amplitude increase were correlated with work load, endurance time and with slope coefficients of RPE-HR linear regression. Symptoms and complaints 24 hours after the task were often localized to the descending part of the trapezius muscle. It is suggested that exertion of the descending part of the trapezius muscle in tasks involving repetitive shoulder flexion may promote discomfort and complaints referred to the neck.     

The impact of ergonomic intervention on individual health and corporate prosperity in a telecommunications environment

This paper is a summary of several papers published in different journals and conference proceedings. The contents deal with the incidence of load related musculoskeletal illness of female workers exposed to various workloads. Furthermore, the changes in workload due to improved workplace design and the effect on the incidence of musculoskeletal illness was evaluated. The sick-leave due to musculoskeletal illness was reduced from 5·3% to 3·1% and the reduction in turn-over from 30·1% to 7·6%. The cost and the benefits of this ergonomic intervention were analysed, and showed that an investment of NOK. 350 000 produced total savings of NOK. 3 200 000.

Postural load was assessed by recording electromyography (EMG) on the upper part of musculus trapezius, postural angles of the upper arm and flexion/extension of the head and back. A quantitative relationship was found between the static trapezius load and the development of musculoskeletal sick-leave, related to the length of employment. Indications were found that certain factors were associated with reduced incidence of musculoskeletal illness. These were:

? increasing the number and total duration of trapezius load below 1% to 2% MVC (Maximum Voluntary Contraction);

? reducing the magnitude of flexion angle of the upper arm in the sagittal plane and distributing the work between flexors and extensors; and

? a more dynamic work pattern of the upper arm.

The relationship between postural load and musculoskeletal injury was studied in comparable groups of female workers with respect to age, working hours per day and time of employment. Psychosocial problems, spare time activities and living habits of the workers did not show any significant differences across the groups. The results from this study indicate that:

? the static trapezius load must be kept at a minimum;

? a median arm flexion should be less than 15° and a median arm abduction less than 10°. These values seem roughly to approach an acceptable arm position; and

? a forward median flexion of the back of less than 20° seems not to lead to a higher rate of low back pain for workers with long periods of employment.     

Influence of mouse position on muscular activity in the neck, shoulder and arm in computer users

Anecdotal reports of shoulder and arm discomfort related to computer mouse use are common. The aim of this study of ten subjects was to examine the influence of mouse position, relative to the keyboard, on shoulder and arm muscular activity and working posture. Surface electromyography and the Rapid Upper Limb Assessment were used to determine the differences in muscle activity and posture during an editing task in each of three mouse positions. Significantly less anterior (p = 0.01) and middle (p = 0.03) deltoid electromyographic activity was demonstrated when the computer mouse was positioned adjacent to a keyboard without a numeric pad when compared to performance of an identical task with the mouse adjacent to a standard keyboard. Electromyographic activity in trapezius muscle did not differ between mouse positions. Working posture of right handed mouse users is improved by removal of the numeric keypad.     

The effects of shoulder posture on neck and shoulder musculoskeletal loading and discomfort during smartphone usage

While using their smartphone, users tend to adopt awkward neck and shoulder postures for an extended duration. Such postures impose the risk of MSDs on those body parts. Numerous studies have been undertaken to examine neck posture; however, few studies have investigated shoulder postures. This study examined various shoulder postures during smartphone use and their effect on neck and shoulder kinematics, muscle loading, and neck/shoulder discomfort. Thirty-two asymptomatic young adult smartphone users randomly performed texting tasks for 3 min at four different shoulder flexion angles (15°, 30°, 45°, and 60°), while maintained a neck posture in the neutral position (0° neck flexion angle). Measures were taken of neck and shoulder muscle activity of the cervical erector spinae (CES), anterior deltoid (AD), upper trapezius (UT) and lower trapezius (LT), and kinematic data (angle, distance and gravitational moment). Results showed AD and LT muscle activity significantly increased when the shoulder flexion angle increased with an opposite effect on CES and UT. A recommended shoulder posture was identified as 30° flexion, as this yielded the best compromise between activation levels of the four muscles studied. This angle also induced the lowest neck/shoulder discomfort score. The findings suggest smartphone users hold their device at approximately 30° shoulder flexion angle with their neck in a neutral posture to reduce the risk of shoulder and neck musculoskeletal disorders when smartphone texting.Relevance to industrySmartphone use in the manufacturing and service industries is an integral part of work and useful means of communication tool. Awkward postures during extensive smartphone use impose an increased risk of both neck and shoulder musculoskeletal disorders. Shoulder flexion angles need consideration when making recommendations about safe work postures during smartphone use.     

An electromyographic study of dental work

Musculoskeletal disorders are common among dentists, and have been ascribed to the demands of high precision work and sustained static loading in the neck-shoulder region, combined with a flexed and rotated cervical spine. In order to determine muscular load levels during dentistry, activity in neck, shoulder, and arm muscles was recorded using an electromyography technique (EMG). Normalized mean, median, 10th and 90th percentile EMG amplitude levels (% maximal reference contraction, %max-RVC) were calculated during ordinary dental work. Among the muscles investigated, the trapezius muscle on both sides had the highest mean (the right trapezius 9.0% and the left 7.6% of max-RVC) and 10th percentile amplitude levels (both about 2% of max-RVC). The trapezius muscles showed similar myoelectric activity on the right and left side, probably because of similar muscular static load on the both sides. The right extensor carpi radialis muscle had a significantly higher muscular load level than the left one, possibly due to stabilization demands on the dominant wrist during demanding precision work. The infraspinatus muscle had low activity level on both sides, reflecting that the dentists worked with a small degree of arm elevation and external rotation. The dentistry work thus seems to generate relatively high muscular load on both trapezius and dominant extensor-carpi-radialis, and relatively low load on the infraspinatus muscle.     

Evaluation of document location during computer use in terms of neck muscle activity and neck movement

This study evaluated the impact on neck movement and muscle activity of placing documents in three commonly used locations: in-line, flat desktop left of the keyboard and laterally placed level with the computer screen. Neck excursion during three standard head movements between the computer monitor and each document location and neck extensor and upper trapezius muscle activity during a 5 min typing task for each of the document locations was measured in 20 healthy participants. Results indicated that muscle activity and neck flexion were least when documents were placed laterally suggesting it may be the optimal location. The desktop option produced both the greatest neck movement and muscle activity in all muscle groups. The in-line document location required significantly more neck flexion but less lateral flexion and rotation than the laterally placed document. Evaluation of other holders is needed to guide decision making for this commonly used office equipment.     

Influence of smartphone use styles on typing performance and biomechanical exposure

Twenty-seven subjects completed 2-min typing tasks using four typing styles: right-hand holding/typing (S-thumb) and two-hand typing at three heights (B-low, B-mid and B-high). The styles had significant effects on typing performance, neck and elbow flexion and muscle activities of the right trapezius and several muscles of the right upper limb (p < 0.0001 by repeated-measure analysis of variance). The subjects typed the fewest words (error-adjusted characters per minute: 78) with the S-thumb style. S-thumb style resulted in similar flexion angles of the neck, elbow and wrist, but significantly increased muscle activities in all tested muscles compared with the B-mid style. Holding the phone high or low reduced the flexion angles of the neck and right elbow compared with the B-mid style, but the former styles increased the muscle activity of the right trapezius. Right-hand holding/typing was not a preferable posture due to high muscle activities and slow typing speed.

Practitioner Summary: Right-hand holding/typing was not favoured, due to increased muscle activities and slower typing speed. Holding the phone high or low reduced the flexion angles of the neck and right elbow, but the former styles increased the muscle activity of the right trapezius compared with holding the phone at chest level.     

Ready steady push--a study of the role of arm posture in manual exertions

This study investigated arm posture and hand forces during bi-manual pushing. Nine male and eight female participants performed isometric exertions at two reach distances (0 and elbow-grip) and six different positions of the hand interface (handle), defined by the plane (longitudinal, lateral, horizontal) and orientation (0 degrees and 45 degrees). Electrogoniometer instruments were used to measure the displacements/postures of the wrist and elbow joints and the forearm, and force measuring strain gauges were used to measure the exerted hand forces (x-, y- and z-components). The results showed that ability to vary arm posture, particularly the forearm, is important during build up of force and that people tend to seek for a balance in the forces applied at the hands by exerting more in the vertical direction. Also, lateral plane handle positions permitted exertion of greater forces than longitudinal and horizontal plane positions.     

Disorders of the cervicobrachial region among female workers in the electronics industry

Ninety-six female employees in the electronics industry were investigated in a cross-sectional study. Symptoms from the neck, shoulder-neck angle, shoulder, arm and thoracic spine regions were coded according to four degrees of severity, and related to individual characteristics. Static muscle strength of five shoulder and arm muscle groups, as well as static endurance were not related to symptoms; nor were medical and work history, productivity, leisure physical activity or hobbies. For the evaluation of postures and movements of the neck, shoulder and upper arm during work a systematic video technique (VIRA) was developed. Large interindividual differences in working technique, and thereby in work load, were obtained. Some of these measures, especially those describing flexion and abduction of the upper arm, were related to symptoms in the neck and shoulder. Stepwise multiple regression analyses confirmed and strengthened these relationships.     

Biomechanical aspects of occupational neck postures during dental work

A typical occupational risk factor for developing neck symptoms is prolonged flexion of the cervical spine. The present aim was to determine joint moments and muscle activity of the neck during forward flexion of the cervical spine to evaluate the load in the neck region. Three dimensional video (3-D) and surface electromyography (EMG) from the splenius muscles were recorded in two common work postures. Using a 3-D static link segment model, moments at the atlanto-occipital (A-O) joint and the seventh cervical-first thoracal (C7-T1) joint were estimated. Maximal extension moments were estimated from maximal neck extension strength. Extension moments at the C7-T1 joint were significantly higher for a highly flexed position (45% of max) compared to a moderately flexed position (32% of max), but remained unchanged at the A-O joint (40% of max). The mean RMS amplitude was 9% of maximal EMG in both positions (no bilateral differences). This difference between mechanical load and muscle load indicates that EMG may seriously underestimate the total loads of the tissue. Lateral flexion influenced the lateral flexion moment while rotation did not influence the rotation moment. The study demonstrates the importance of quantification of joint loads in occupational risk assessment of the neck.

Relevance to industry

3-D biomechanical calculations provide information on the mechanical load during work. Because EMG may underestimate total tissue load, calculations of joint moments in combination with information on muscle activity and strength are necessary to estimate different tissue loading of significance for overall risk identification.     

The strain on the shoulder and neck muscles during letter sorting

Shoulder muscle strain was tudied during letter sorting in 10 healthy, experienced male mail carriers. Muscular load on the deltoid, trapezius and infraspinatus muscles was evaluated during a normal working day at the post office by vocational electromyography. To asses electromyographic indications of muscle fatigue the letter sorting was interrupted by a series of test contractions in six of the subjects. The study revealed an unacceptable static work load on the muscles during letter sorting. Electromyographic indications of muscle fatigue were present in the trapezius and infraspinatus muscles in three of the six subjects probably caused by sustained, elevated arm work. Recommendations for an improvement of the work place design are finally given.     

Physical workload in neck, shoulders and wrists/hands in dental hygienists during a work-day

Physical workload was recorded by electromyography, inclinometry and goniometry for twelve female dental hygienists during authentic work. Their work was, in relation to other types of work, characterised by pronounced head flexion (90th percentile 46°), high loads on the forearm extensor muscles (90th percentile 23% and 18% of maximal EMG (MVE), for the right and left sides, respectively), average loads on trapezius muscles (90th percentile 15% and 14% MVE), average arm elevation (99th percentile 83° and 72°) and average wrist flexion and velocities (50th percentiles 17° of extension and 7.3°/s, for the right side). Manual scaling and machinery (use of ultrasonic scaling and hand-pieces) showed higher loads on the trapezius muscles, regarding muscular rest, as well as the 10th and 50th percentiles, than the other tasks, and for the forearm extensor muscles, an almost complete lack of muscular rest (0.1% time), and much higher loads regarding the 10th and 50th percentiles. Further, more pronounced head flexion and lower head and upper arm velocities were found, indicating more constrained postures for the neck and shoulders for the manual scaling and machinery. Use of ultrasonic scaler reduced the 50th percentile loads on the right forearm extensor muscles, but had no effect on the fraction of muscular rest and on the 10th percentile load. These findings are consistent with the high prevalences of musculoskeletal disorders among dental hygienists.     

Activity in neck-shoulder and lower arm muscles during computer and smartphone work

Relevance to industryThere is emerging evidence of musculoskeletal problems related to smartphone work and a rapid transition to mobile workplaces, where smartphones are key working tools. Aim: The study's aim was to compare muscle activity during computer work with smartphone work and to see what possible effects ergonomic recommendations for smartphone usage have on muscle activity.MethodsActivity was measured bilaterally from the upper trapezius muscle and from lower arm muscles on the right hand side, on twelve participants with surface electromyography who performed e-mail work on the computer using ergonomic recommendations, smartphone in a self-chosen way of working and on smartphone with ergonomic recommendations. Effects on productivity was not assessed in this study.ResultsActivity in m. trapezius and m. extensor digitorum was significantly higher during computer work (p < 0.05) than during the two smartphone usages and activity in m. interossei dorsalis 1 was, vice versa, significantly higher during smartphone work (p < 0.05). Comparison of smartphone in self-chosen way of working and smartphone with ergonomic recommendations showed no significant differences.ConclusionsPrevious research has highlighted the benefits of variation of work postures. This paper indicates that replacing the computer with a smartphone gives the trapezius muscle an opportunity to rest.     

Bowing with a trunk flexion of 90°: Perspectives on consumers and service personnel

In this study, we analyzed 100 questionnaire responses to determine consumer conceptions of service personnel bowing as a gesture of politeness. In addition, 12 female participants demonstrate five distinct degrees of bowing by bending their trunk 30°, 45°, 60°, and 90° from an upright position; their spinal curvature and trunk muscle activations were measured. The results showed that most respondents believed that bowing at 30° was the most satisfactory angle, followed by 45°. Bowing at 45° or 60° caused the survey respondents to feel honored, whereas bowing at 90° induced feelings of surprise and novelty; however, this angle produced the lowest proportion level of agreements on the items at ease, necessary, and appropriate among the 100 respondents. When the 12 participants bowed at an angle ≤60°, their spinal posture became increasingly kyphotic, and their muscle activations became more pronounced. Myoelectric silencing of the erector spinae was observed when the participants bowed at 90°, which can increase the risk of low back injury. These findings provide service industries with a reference for the design of service encounters.Relevance to industryBowing has traditionally signified politeness and respect in many countries. However, whether bowing at a greater trunk flexion actually satisfies customers remains unclear. Findings from this study clarify the customer perceptions and body loads of service personnel when bowing at various trunk positions. The extreme bowing postures (e.g., 90° trunk flexion) should be avoided.     

Effects of arm support on shoulder and arm muscle activity during sedentary work

The aim of this study was to evaluate different arm supports by comparing the activity of shoulder and arm muscles during various work tasks, with and without the lower arm supported. Twelve female subjects, aged between 23 and 37 years, were asked to perform three types of tasks: typing, simulated assembly work (in two different positions), and pipetting. The supports used were: fixed arm support (FIX), horizontal movable arm support (HOR), and spring-loaded arm support (SLA). During the experiments, the electromyograms (EMG) of four muscles were simultaneously recorded: m. deltoideus anterior and lateralis, m. trapezius pars descendens and m. extensor carpi radialis brevis. Normalization was made against maximum isometric contraction. The mean values of the normalized EMG levels showed a reduced EMG level of the shoulder muscles when using arm supports in all the tasks, and for all muscles but the wrist extensor, compared to the EMG levels without arm supports. The horizontal movable support was more effective in reducing the EMG levels of the shoulder muscles than other arm supports, in tasks at table height. Thus, it is possible to reduce muscle activity of the shoulder region by using arm supports. Further research is needed to make biomechanical calculations to compare the EMG level of these muscles using suspension and the effects of inclination of work task.     

Myoelectric arm using artificial neural networks to reduce cognitive load of the user

Today’s multiple degree-of-freedom myoelectric prosthesis relies only on direct control by the processed electromyographic signal. However, it is difficult for the wearer to learn unnatural muscle contractions in order to wield more than three DoFs of the arm. This makes it almost impossible to use more complex prostheses with a larger number of actuators. Methods based on sensor–actuator loop and artificial intelligence may reduce cognitive load of the user by removing low level control, and an intelligent control system would make it needless to micromanage every action. For this purpose, sensor system for body segments motion capture was developed, as well as sensor system for prosthetic limb’s environment motion capture. Neural networks were designed to process data from the sensor systems. For the identification of the knee angle, orientation trackers were used. Neural network predictor of arm positions predicts the shoulder angle using the information about movement of the lower limb. In the case of the periodic/cyclic movements of the legs, such as walking, the control unit uses typical movement patterns of the healthy upper limb. Ultrasonic range sensors are used to create 3D map of objects in the environment around the arm. Neural network predictor of object positions predicts collisions. If the potential collisions are identified, the control unit stops arm movement. The new methods were verified by MATLAB and are designed as a part of assistive technology for disabled people and are to be understood as an original contribution to the investigation of new prosthesis control units and international debate on the design of new myoelectric prostheses.